Search

The western Tibet Plateau comprises a series of crustal terranes that were successively accreted to the southern margin of Eurasia attending the transition of the Palaeo- to Neo-Tethys. We present the first detailed SHRIMP zircon U–Pb chronology, major and trace element, and Sr–Nd–Hf isotope geochemistry of three Cretaceous plutons (Kalaqigu, Aranbaotai, and Hongqilapu) from this area. SHRIMP zircon U–Pb dating shows that the three plutons were emplaced in the Early Cretaceous. The 114 Ma Kalaqigu pluton is composed of strongly peraluminous high-K calc-alkaline monzogranite and granodiorite. These rocks consist of plagioclase, K-feldspar, quartz, and biotite, and show variable Sr–Nd and Hf (in situzircon) isotopic compositions [87Sr/86Sr (T) = 0.7098–0.7157, ϵNd(T) = –8.1 to –15.6, and ϵHf(T) = –10.2 to –19.8 (mean –14.8)]. Elemental and isotopic data suggest that the granitoids were generated by partial melting of metasedimentary basement in the normal to thickened lower-crust triggered by underplating or injection of hot mantle-derived magmas in a continental arc setting. The 110 Ma Aranbaotai pluton consists of strongly peraluminous high-K calc-alkaline two-mica monzogranites. These rocks are composed of K-feldspar, plagioclase, quartz, biotite, and muscovite, and also show variable Sr–Nd and Hf (in situzircon) isotopic compositions [87Sr/86Sr (T) = 0.7075–0.7144, ϵNd(T) = –9.4 to –13.3, and ϵHf(T) = –5.4 to –16.4 (mean –10.0)]. Elemental and isotopic data suggest that the granites were formed by partial melting of subducted sediments at a depths of <40 km to ∼40–50 km in the collision (overthrust) zone during continental collision. The 102 Ma Hongqilapu pluton consists of metaluminous high-K calc-alkaline granodiorites. These rocks are composed of plagioclase + K-feldspar + quartz + amphibole + biotite ± clinopyroxene, and show uniform Sr–Nd and Hf (in situzircon) isotopic compositions [87Sr/86Sr (T) = 0.7078, ϵNd(T) = –7.6 to –8.0, and ϵHf(T) = –7.2 to –11.2 (mean –9.4)]. Elemental and isotopic data suggest that the granitoids were generated by crystal fractionation of basaltic magmas in a continental arc setting. Our new data suggest that at 114 Ma, the Karakorum terrane was still an active continental arc fuelled by the northword slab subduction of the Mesozoic Tethys; collision of the Kohistan terrane with the Karakorum terrane most likely occurred at 110 Ma, resulting in the closure of the Mesozoic Tethys; since 102 Ma, the southern Karakorum terrane hosted to a new continental arc coupled with the northward slab subduction of the Neo-Tethys. [ABSTRACT FROM PUBLISHER]

The Jiangnan Orogen is located at a key tectonic position along the junction between the Yangtze and Cathaysia blocks. We obtained detailed major and trace elements, whole-rock Nd + zircon Hf isotope data, and U–Pb age data from several Mesozoic granites, including the Fuling (FL), Taiping–Huangshan (TH), Lingshan (LS), Sanqingshan (SQS), and Baijuhuajian intrusions in order to investigate their sources and petrogeneses related to extension in South China. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analyses of zircon from the FL, TH, SQS, and LS bodies yield Early Cretaceous ages of 124–135 Ma. These plutons are alkali-feldspar granites to syenogranites–monzogranites, and show A-type affinities. They have high K2O and total alkali contents, and are enriched in rare earth elements (except for Eu), Zr, and other high-field-strength elements as well as high Ga/Al ratios, and are depleted in Ba and Sr. These granites are metaluminous to weakly peraluminous (ACNK from 0.81 to 1.27). The whole-rockϵNd(T) values of −5.34 to −0.96 are coupled with zirconϵHf(T) values (from −5.3 to +4.24), and all samples plot along the mantle array. Field observations, geochronology, geochemistry, Nd isotopic, and zircon Hf isotopic compositions suggest that they formed by the partial melting of Mesoproterozoic metamorphic basement, with input from juvenile, mantle-derived materials in the shallow (<30 km) crust at high temperatures (756–965°C). These melts underwent crystal fractionation of biotite, plagioclase, and K-feldspar. The upwelling of asthenosphere triggered partial melting of the metamorphic protolith in a back-arc or intra-arc rift setting, reflecting rollback of the Pacific plate. Our research adds new geochronologic constraints on Cretaceous (135–120 Ma) A-type granites from the NE sector of the Jiangnan Orogen. Combined with previous research, we suggest that three main episodes of late Mesozoic extensional tectonism took place in South China: (1) 190–170 Ma (mainly inland), (2) 165–120 Ma (including 165–150 Ma in SE Shi-Hang, 135–120 Ma in NE Shi-Hang, and ∼125 Ma in the Lower Yangtze River Belt), and (3) 100–90 Ma (coastal area), showing an oceanwards younging trend due to the subduction of the Palaeo-Pacific plate. [ABSTRACT FROM AUTHOR]

Early Cretaceous felsic intrusions and volcanic rocks are widespread in the Gan-Hang Belt, SE China. In this study, we report a distinctive high-Zr rhyolite (802–1145ppm Zr) from the Dazhou uranium district in the eastern Gan-Hang Belt. SHRIMP zircon U–Pb dating shows that the Dazhou rhyolite erupted at 127.3±1.7Ma. Geochemical data indicate A-type characteristics for the Dazhou rhyolite, such as high contents of alkali elements, high and high Ga/Al ratios, enrichment in some LILEs, HFSEs, and REEs, and depletion in Sr, Ba, P, and Ti. Compared to other A-type granitoids reported previously in the same belt, the Dazhou high-Zr rhyolite shows only a slight enrichment in other HFSE elements and LREE, but has similar whole-rock Nd and zircon Hf isotopic compositions to those A-type granitoids with relatively lower Zr concentrations, indicating that all these A-type rocks may have similar magma sources. Hence, the difference of Zr contents in these rocks is not due to their different source rocks. We found that zircon grains in the Dazhou high-Zr rhyolite are generally inheritance-poor, and the calculated zircon saturation temperatures are extremely high at ∼1000°C. It is therefore suggested that the distinctive high-Zr characteristic of the Dazhou rhyolite was a result of high-temperature suppression of zircon crystallization by ambient mafic magmas correlative with the upwelling of the asthenospheric mantle. The fundamental difference in magma temperature and magmatic evolution mechanism between the Dazhou high-Zr rhyolite and other A-type granitoid suites in the eastern Gan-Hang Belt brought about their different Zr concentrations. [ABSTRACT FROM AUTHOR]

Abstract: The widespread occurrence of Mesozoic granites in the Gan-Hang Belt in Southeast China is associated with similarly widespread, economically important mineralization, but the precise timing, origin, and plate tectonic significance of these granites are not well understood. We have studied two of these (Early Cretaceous) granite bodies, the Yangmeiwan granite and the Daqiaowu granitic porphyry, from the northeastern part of the Gan-Hang Belt in western Zhejiang Province by zircon U-Pb geochronology, major and trace element analyses, and Nd–Hf isotopic analyses. LA-ICP-MS and SHRIMP U–Pb dating of zircon grains from these two granites yield ages of 133–136Ma, representing a Early Cretaceous magmatic event. These granitic rocks are metaluminous to weakly peraluminous and have a pronounced A-type geochemical signature with high Na2O+K2O, Fe2O3*/MgO and Ga/Al ratios. They show low CaO, MgO and TiO2 contents, enrichment in some LILEs (such as Rb and Th) and HFSEs (such as Zr, Y), depletion in Sr, Ba, P, Eu and Ti. They also show A2 subtype affinity and were probably formed at a high temperature (~810°C for Yangmeiwan granite and ~850°C for Daqiaowu granitic porphyry). These A-type granitic rocks show bulk rock εNd(t) values in the range of −6.5 to −3.6 and zircon εHf(t) values from −7.8 to −0.9, with Mesoproterozoic TDM c ages for both Nd and Hf isotopes. Geochemical and isotopic data suggest that these A-type granitic rocks were generated largely by partial melting of granulitized Mesoproterozoic metamorphic basement rocks (including parametamorphic and orthometamorphic rocks), with a possible input of mantle-derived materials and followed by subsequent fractional crystallization. Our new data together with previous published data suggest that the Cretaceous A-type granitic rocks with ages between 137Ma and 122Ma occurred along the Gan-Hang Belt, indicating an important Mesozoic extensional event in Southeast China. This event represents either a back-arc extension or an intra-arc rift due to the roll-back of the paleo-Pacific plate, and it started as early as ~137Ma in Xiangshan and Xinlu basins. It is earlier than ~130Ma as previously suggested in the studied region and later than that along the south part of the Shi-Hang Zone. With ongoing extension during slab roll-back of paleo-Pacific plate, the crust and lithospheric mantle became progressively thinned. The upwelling of asthenosphere would have triggered crustal rocks to partially melt, generating granitic magmas. Our new geochemical data also suggest that mantle-crust interaction in the formation of the A-type granitic magma along the Gan-Hang Belt gradually intensified from early to late stages between ca. 137 and ca. 122Ma. [Copyright &y& Elsevier]